In the heavy duty pneumatic tire, a belt is generally arranged on an outer peripheral side of a crown portion of a carcass for strengthening a tread portion. As the belt is mainly used a cross belt comprised of plural belt layers, cords of which layers being crossed with each other, or a slant belt comprised of one belt layer containing cords inclined in one direction with respect to a circumferential direction of the tire.
When an air pressure is filled in such a pneumatic tire, the hoop effect of the belt becomes smaller in a shoulder portion of the tread positioning cut ends of the belt cords than a central portion of the tread, so that the size growing amount of the belt at the tread shoulder portion becomes larger than that at the tread central portion. As a result, a portion of a tread rubber corresponding to the tread shoulder portion is deformed so as to relatively largely increase the size as compared with the other portion, and there is a problem that separation failure is easily caused between the belt and the tread rubber.
In tires having an aspect ratio of not more than 60%, the size growing amount of the belt particularly tends to become larger in the tread shoulder portion.
As a countermeasure for suppressing the size growth of the belt in the tread shoulder portion, it is proposed and used to arrange a circumferential strengthening layer containing wavy or zigzag cords extended in the circumferential direction at an inner peripheral side or an outer peripheral side of the belt or between the belt layers. In this case, the size growing amount of the belt in the tread shoulder portion can be suppressed to prevent the separation failure and improve the durability in the tread shoulder portion.
In the tire having the above construction, however, there is caused a new problem that since the bending rigidity of the circumferential strengthening layer in a widthwise section is small though the separation failure resulted from the size growth of the belt in the filling of the internal pressure can be suppressed, if the tire rides on a projection such as stones or the like during the running, the belt and the circumferential strengthening layer are deformed so as to make a recess with a deep inclination angle with respect to the widthwise direction starting from a position of a side edge of the circumferential strengthening layer and hence widthwise shearing strain produced in the belt increases and strain and stress are concentrated to the side edge of the belt to generate a separation failure.
In order to solve this problem, JP-A-2000-62411 proposes a pneumatic tire in which the circumferential strengthening layer is arranged at the inner peripheral side of the belt and a belt reinforcing layer having a width wider than that of the circumferential strengthening layer is arranged at the outer peripheral side of the belt and the cord angle of the belt reinforcing layer with respect to the circumferential direction of the tire is made larger than the inclination cord angle of the belt with respect to the circumferential direction of the tire.
According to this proposal, the belt reinforcing layer largely contributes to increase the bending rigidity in the widthwise section of the tire, so that even if the tire rides on the projection such as stones or the like during the running, the displacements of the belt and the circumferential strengthening layer at the position ride on the projection inward in the radial direction are made small and the starting point of the deformation is moved up to the side edge of the belt reinforcing layer, whereby the distortion deformation of the belt and the circumferential strengthening layer can be made at a shallow inclination angle with respect to the widthwise direction. Thus, the widthwise shearing strain produced in the belt can be suppressed to prevent the separation failure resulting from the concentration of strain and stress to side edge of the belt.
In such a tire, however, there is a problem that the weight of the tire as a whole is too large because the circumferential strengthening layer is arranged at the inner peripheral side of the belt and further the belt reinforcing layer is arranged at the outer peripheral side of the belt. Also, it is necessary to consider the separation failure between the belt and the belt reinforcing layer in accordance with the further reduction of the aspect ratio in recent tires.
The invention is to solve the above problems of the conventional techniques and to provide a heavy duty pneumatic tire capable of preventing the deterioration of the durability at the position of the widthwise side edge of the belt without increasing the weight of the tire as a whole in the structure having an arrangement of the strengthening layer for enhancing the rigidity in the circumferential direction and even in a tire having a low aspect ratio.